Electrophotographic techniques and processes for image reproduction are well known in the art. These generally involve use of an electro-photosensitive and electro-charge-dissipatable base upon and by which the image is retained and fixed by sequential steps that generally (and in simplest and most fundamental terms and concepts) include:
1. Sensitization or activation of the image-retaining electrophotographic base by charging, usually at uniform level over and in the entire surface, of its generally insulated photoconductive and charge-retaining surface or image bearing portion by electrostatic or equivalent means;
2. Imparting a latent image in the charged base by exposing it to a charge-dissipating influence, such as light, in such a manner as to obtain an image configuration (i.e., light and dark or shadow effects, as it were) influence or reproduction potential and capability therein whereby and wherein gradient or varying charge intensities and levels in configuration conforming relationship to the image being reproduced are realized by selective dissipation on or at any part or portion of the electro-photoconductive surface according to the relative intensity of the charge dissipating influence to match the desired image cast upon each given part or portion of the surface; then
3. Developing and fixing the exposed surface by depositing on the imaged layer a charge-responsive dye or pigment colorant (which includes black) or developing material, generally called a "toner," which remains in varying concentration or color-producing disposition according to the level of charge dissipating at any given part or portion of the electro-photoconductive surface in conformance with the latent image imposed thereon so that permanent reproduction of the image is achieved and obtained.
The phenomenon in its various techniques and aspects is well known and understood in the art and has been extensively described, in vast quantity and proportion, in considerably numerous patent and other literature sources. As cursory evidence of this reference may be had to U.S. Pat. Nos. 3,052,539; 3,249,430; 3,259,581; 3,383,209; 3,595,691; 3,660,086; 3,751,247; 3,758,305; 3,802,880; and 3,809,555 and the citations therein amongst the multitudinous additional teachings and disclosures available in the art (such as those included, inter alia, in International Search Classes GO3f, GO3g and so forth). Thus, no further fundamental elucidation or detailed description is necessary or required as to same for full comprehension and clear understanding of the present invention.
The present invention is generally applicable to electrophotographic reproductions that are made in either black (or other monochromatic color)-and-white but not well adapted for multiple or full color using any electro-photoconductive base that is suitable and satisfactory for the purpose (including, of course, the very popular paper composite bases even though other support substrate materials can be employed) that is capable of yielding either the so-called -- and well understood in the art -- continuous tone reproductions as well as with and for bases that are effective to make half-tone reproductions.
In this connection, it is well known and understood in the art that: (i) a continuous tone reproduction is an image, that is comprised of gradient tones ranging from a relatively absolute black to a relatively absolute white (with the equivalent analogy applicable when colors other than black are involved); while (ii) a half tone reproduction is an image that is comprised (either in black-and-white or color) or dots (or the like resemblances) which may vary in dot density distribution over a given area or in dot count taken along a given linear length and which are dissociated and distinct from one another and, in general, are of varying diameter. Pertinent background information which yields clear understandability of various aspects and definitions of the characteristics and features involved in and relevant to both continuous tone and half tone image reproductions may be found at pages 1,372 through 1,381 and 3,218 through 3,220, respectively, of the authoritative "Encyclopedia Of Photography," 1967 Edition, published by the Greystone Press of New York City.
Further along this line, a good standard reference in the electrophotographic field is "Xerography And Related Processes" by Dessauer and Clark, 1965 Edition, published by Focal Press.
The electro-photoconductive base, depending on its particular composition, construction and chracteristics, is ordinarily sensitized for image capture thereon by charging the electro-conductive layer thereof to a relatively uniform voltage intensity (without limitation in the indicated range) gradient from its lowermost to its uppermost image-retaining surface of between about 200 or less and about 1000 or more volts, with 400-700 volts being a common charge level encountered to provide the electrophoretic effect voltage retention and capability necessary to have for selective, image-conforming charge dissipation when subject to a given activating influence.
This induced charge will gradually and eventually dissipate with passage of time, the leakage rate being found to vary not only with the level of the induced charge but with the charge-retaining characteristics of any given base. Thus starting at the fully charged point in time, without any charge-dissipation caused by the activation influence, the base will decay to a literal "no-charge" (or inadequately charged level for effective image retention). In the ordinary printing procedure, it is generally advantageous for the totality of the activating influence to reach the proper degree of exposure to be applied well ahead of the ineffective decay point that is reached in normal charge loss dissipation on standing of the base without any image-impressing consequence of subjection to the activating influence. In fact, it is necessary that this be done before the normal decay point is reached if effective image fixing is to be achieved.
In this connection, it must be taken into account that various continuous as well as half tone reproducing capable electro-photoconductive bases exhibit various degrees or levels of "sensitivity" with respect to their capability of being activated so as to render and yield the desired tone and resulting image-retaining effect. This sensitivity can be measured and expressed by the number of steps reproducible on and discernable after exposure and fixing of the image in the base by comparisons made with and to the well known and standard Stouffer Grey Scale. The Stouffer Grey Scale has 21 light transmitting steps in the sequence of which maximum light transmission (or transparency) is Step No. 1 which is near if not at 100% in light transparency capability and minimum light transmission (or opacity) is Step No. 21 which is near if not at 0% in light transmitting capability, with gradations in light transmitting capability evident between 100 and 0% on a descending scale in intermediate Steps 2 through 20. An excellent quality reproducing base generally encompasses at least Steps Nos. 10 or 12 through 21 on the Stouffer Grey Scale, although lesser numbers of light transmitting capabilities according to Stouffer Grey Scale step effectiveness and measure can be utilized insofar as concerns sensitivity of a given base to render it in effect a satisfactory or at least operable image receiver and reproducer. Of course, reproduction capability according to the higher numbered steps is necessary in order to define and pick out the dark areas in a given reproduced image, whereas and conversely, some reproduction capability in the lower numbered Steps is mandatory for white or light areas to be discernable and copied in the reproduced image.
Thus, a base sensitive to and capable of operating in and over only five or six Grey Scale Steps may yield a satisfactory or even good image receiver, especially if the given limited number of Steps are well dispersed over the entire scale. In general, along this line, the reproduction to be of decent continuous tone quality should be capable of image reproduction so as to include: (i) for the "white" areas according to, say, at least one of the steps in the Steps Nos. 1-3 range of the Scale; (ii) for the "flesh and middle tone" areas at least one of the steps in the Steps Nos. 5-7 range of the Scale; and (iii) for the "dark and denser" areas of the reproduced image at least one of the steps in the Steps Nos. 11-21 range of the Scale. Accordingly, while it is conceivable to have a product which responds and corresponds to only two widely separate Steps, it is usually the case that the sensitivity of the base should be such that it is capable of responding to and reproducing at least three of the Steps of the Scale dispersed in the above-indicated range portions of the Scale.
As particularly appropos illustrations of suitable and relatively typical electro-photoconductive base materials and their composition and construction and colorants (such as "toners") and their formulation(s) which are more or less suitable for use and application, after appropriate adaptation, in and for practice of the present invention, reference may be had in addition to that indicated in the above-noted citations to the teachings and disclosures of U.S. Pat. Nos. 3,249,430; 3,259,581; 3,802,880; 3,804,619; and 3,809,555 plus Canadian Pat. Nos. 846,740; 846,741 and 846,742.
The surface of the image-retaining base is usually sensitized prior to exposure by means of subjecting it to electrical energizing fields, such as those provided by corona discharge means and effects (although, here too in other given systems other functionally equivalent sensitizing influences are adapted to and capable of employment).
Ordinarily, the latent-image retaining exposed base is most advantageously subject to the development and toning (or colorant-affixing) procedure and sequence that is utilized as soon as possible after exposure. Although some delay in the development after exposure is tolerable, the development or image-fixing procedure must perforce be commenced before dark decay of the base (by charge dissipation or leakage loss) becomes a problem. As is well known, the time for dark decay to occur after sensitization (and subsequent exposure) of a electrophotoconductive base varies with any given base according to its charge-retaining capabilities. Thus, knowing the decay rate of any given base allows one to determine what time period can be safely and acceptably tolerated between sensitization and exposure followed by development. Usually, at least about 11/2 to 3 or 4 minutes (and sometimes as much as 5 to 10 or even more, depending on particularities of the base being utilized) is available between sensitization and image-fixing by development before dark decay goes to an undesirable extent (that ordinarily being a point at which the charge acceptance capability of the base has suffered a loss in the neighborhood of a 20-25% or so decrease).